An induction motor among motors is driven relatively easily during operation from its start-up, and further can be used by just being directly connected to a commercial power supply, so that it has been widely employed for various uses.
With recent development of power electronics, it is possible to drive the induction motor at variable speed. That is, alternating-current power is once converted into direct-current power, and by an inverter that inversely converts the direct-current power into alternating-current power different in frequency by switching a semiconductor switch with high frequency, the induction motor is driven. However, a fan motor, a compressor, a motor of a pump, or the like drives an object at a constant rotational speed, and thereby purposes of their usage are sufficiently satisfied in many cases. In a factory or the like in particular, the proportion of a motor directly driven by a power supply frequency of 50 Hz, 60 Hz, or the like is large by reason of the increased number of installed motors • the size of capacity, and the like and due to problems of cost • installation space of the inverter, and the like.
On the other hand, as for reactive power, such as the induction motor, there are a lot of electrical apparatuses taking current with a lagging power factor, and when the power factor is low, current supplied to a load is increased. Thus, capacity and loss of “a transmission distribution installation such as a transformer” existing in a distribution line or a distribution system are increased. Further, in a transmission distribution system in general, system reactance at a customer side is inductive due to reactance existing in a wiring, leakage reactance of a transformer, or the like. Thus, voltage at a receiving end of a customer reduces by current with a lagging power factor.
For a problem caused by the existence of such a load with a lagging power factor, compensation by a device to generate reactive power has been performed.
As the easiest method for performing such compensation, there is a method of connecting a phase advancing capacitor in parallel to a power supply. The above method is to apply power to an electric power system in stages by using a switch so as to make capacitance of the capacitor become an appropriate value to thereby generate reactive power corresponding to a load. Further, as one with large capacity to be connected to an electric power system, there is a synchronous phase modifier. The synchronous phase modifier controls a generation amount of reactive power in a manner that a synchronous machine being a rotary machine is connected to the electric power system to control a field current of the synchronous machine. In recent years, in an electric power system or for a very variable load, there has been used a static-type reactive power compensation device to which a semiconductor power conversion technique is applied in order to stabilize voltage, or the like (a TSC, a TCR, an SVG, an STATCOM, and so on have been known).
A magnetic energy recovery bidirectional current switch disclosed in Patent Document 1 and the like, (which is described as “a magnetic energy recovery switch” here), is a switch circuit configured with a bridge circuit composed of four reverse conductive semiconductor switches and a capacitor connected between direct-current terminals of the above bridge circuit. As a technique to improve a power factor of a load by using such a magnetic energy recovery switch, there is a technique described in Patent Document 2. In the technique described in Patent Document 2, the magnetic energy recovery switch is connected in series between an AC power supply and an AC load to be switched according to a cycle of the AC power supply, and thereby the magnetic energy recovery switch operates as a series capacitor to improve a power factor. Further, Patent Document 3 discloses that such a circuit is applied to a rotary machine such as an electric motor or a power generator to thereby improve a power factor of the rotary machine in which an inductance such as a leakage inductance exists.
Applying the magnetic energy recovery switch to an AC load with a lagging power factor makes it possible to generate reactive power to a power supply, and by the above reactive power, reactive power necessary for the load with the lagging power factor can be compensated. Patent Document 4 discloses that the above fact is employed, and the magnetic energy recovery switch is applied to one of two AC loads with a lagging power factor to make the entire power factor become one, and thereby the power factor is improved.
The induction motor directly driven by a commercial power supply is simple but has problems on its efficiency at the time of operation, excessive starting current, and the like because it is not electrically controlled.
On the other hand, as for the compensation of reactive power, compensation of reactive power at a receiving point of an electric power system or a customer installation has been performed widely. However, in a large-scale customer, a path from a receiving point to an actual load is long in many cases. Thus, flowing of current with a low power factor is disadvantageous in terms of loss of a distribution line or capacity of an installation. It is necessary to install a reactive power compensation device in the vicinity of a load in order to perform compensation of reactive power in the vicinity of the end of the distribution line.
Here, the previously described static type reactive power compensation by a semiconductor converter widely used currently will be described. The TSC is to switch a capacitor by a thyristor switch. Thus, it is not possible for the TSC to continuously control reactive power at a high speed. The TCR is the combination of a fixed capacitor and a reactor controlled by a thyristor. Thus, the TCR has a problem that not only the capacitor but also the reactor is needed. The SVG switches a reverse blocking type semiconductor switch at a high speed in order to perform PWM and the like. Thus, the SVG has a problem that switching loss increases. Accordingly, it is not common to employ these conventional reactive power compensation devices for compensation of reactive power in the vicinity of the end of a distribution system.